The effects of metallicity and magnetism on the radii of M dwarf stars
Kesseli, Aurora Yvonne
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M dwarfs are ubiquitous in the galaxy, yet their fundamental properties are not precisely known. Radii are particularly difficult to determine because M dwarfs are intrinsically small and faint, leading to only a few radius determinations using either long-baseline optical interferometry or eclipses of binary stars. Observations rarely agree with models, and the scatter in M dwarf radius relations is significantly larger and less understood than it is for higher mass stars. I explored the two main hypotheses evoked to explain discrepancies between model radii and observed radii, namely effects from metallicity and strong magnetic fields. I conducted a spectroscopic survey of M dwarfs with a wide range of metallicities and derived radii using the Stefan-Boltzmann law in order to constrain radius relations for the lowest mass and lowest metallicity stars. I found that solar metallicity stars can be up to five times larger than their low-metallicity counterparts for a given effective temperature, but that metallicity has a relatively small effect on mass- or luminosity-to-radius relations. To test the effect of magnetism on radii, I determined a statistical distribution of radii for magnetically active M dwarfs by combining measured rotational broadening values with literature rotation periods. I found that the magnetically active stars were on average 10-15% larger than model predictions and that models and observations were most discrepant for the lowest-mass stars. To deduce whether the 10-15% radius discrepancy could be due entirely to the spotted nature of these stars, I determined the spot temperature and spot filling fraction of one of the most magnetically active stars in my sample. I measured a high spot filling fraction, spot temperatures several hundred Kelvin lower than the photosphere temperature, and I also detected evidence of faculae on the stellar surface. I concluded that spots are the primary cause for models overestimating the sizes of low-mass stars, and that stellar-evolution models should consider the effects of spots to more accurately predict the sizes and temperature of all M dwarfs.
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